Vehicle electrics
Powering a van conversion
A van electrical system is built outward from the battery, and every part after it is sized by a number the battery does not publish. Here is the chain, in order.
By Jean-Benoit Buisson
Everything is sized by the battery
Not by its capacity — by its maximum current. That figure decides the cable, the fuse, the busbar and the isolator, and it is the one number a battery listing is least likely to publish clearly.
Find it first.
1. The battery
Capacity in amp-hours, chemistry, and the maximum continuous discharge current.
For lithium, one more: the low-temperature charging cut-off. Charging a lithium cell below freezing damages it permanently, and a battery that refuses is protecting itself. Not every listing says whether it does.
2. The DC-DC charger
Between the vehicle battery and the leisure battery, always. It takes whatever the alternator produces and makes a proper charging profile from it.
Sized by charge current, and constrained by what the vehicle wiring will carry. A 40 A charger on thin existing wiring is a 40 A charger in a smaller circuit.
3. The busbar
Where everything meets. Positive on one, negative on the other, and every load and source lands there instead of stacking six ring terminals on a battery post.
Sized by the total current, and the stud size decides which lugs fit. Copper conducts better than brass, which at high current stops being academic.
4. The isolator
One switch that kills the whole system. For maintenance, for storage, for the day something goes wrong.
Sized by continuous current, with a surge figure alongside it — and the two are different numbers. A switch published with only one leaves you guessing which it meant.
5. The inverter
Only if mains is genuinely needed. Everything a van actually uses runs on twelve volts, and each conversion loses something.
Pure sine or modified is the first question, and idle consumption is the second — an inverter left switched on draws its idle current continuously whether or not anything is plugged in.
6. The monitor
A shunt, not a voltmeter. Voltage tells you roughly where a battery is; a shunt counts what actually went in and out.
On a lithium battery, where voltage is nearly flat across most of the discharge, a voltmeter is close to useless and a shunt is the only honest answer.
The part this guide keeps returning to
Cable and fuses. Every step above needs both, sized to the current at that step, and nothing in a van electrical system fails as quietly as an undersized conductor.
